1. Field of the Invention
The present invention relates to olefinic thermoplastic elastomer blend compositions which are adherent to metal, glass, wood, polyolefins, and polar polymers with no pretreatment or use of other adhesives.
2. Description of the Prior Art
Polyolefins are polymers of relatively simple olefins such as ethylene, propylene, butene(s), isoprene(s), and pentene(s) and include copolymers and modifications as disclosed in Whittington's Dictionary of Plastics, p. 252 (Technomic Publications 1978). An undesirable characteristic common to all polyolefins is a non-polar, non-porous inert surface which does not adhere to metal, glass, polar plastics, and other surface coating and adhesives materials without proper priming or special pretreatment.
An "elastomer" is a rubber-like polymer which can be stretched under tension to at least twice its original length and retracts rapidly to its original dimensions when the tensile force is released. An elastomer has an elastic modulus less than about 6,000 psi and an elongation generally greater than 200% in the uncrosslinked state at room temperature in accordance with the method of ASTM D412.
A "thermoplastic" material is a linear or branched polymer which can be repeatedly softened and made flowable when heated and returned to a hard state when cooled to room temperature. It generally has an elastic modulus greater than 10,000 psi in accordance with the method of ASTM D638. In addition, thermoplastics can be molded or extruded into articles of any predetermined shape when heated to the softened state.
Thermoplastic elastomers (TPE) are a family of materials that have the properties of elastomers but can be processed like thermoplastics. When TPEs are made from polyolefins as described above, they are known in the industry as thermoplastic olefin elastomers (TPO). TPEs and TPOs are generally made by blending two or more polymers or by synthesizing block copolymers or graft copolymers. In each case the thermoplastic elastomer contains at least two segments, one being a rigid, usually semi-crystalline thermoplastic and the other being an amorphous elastomer.
The blending technology for manufacturing TPOs has been well known in the industry for many years. These polymer blends can achieve unique properties which bridge the gap between olefinic elastomers and polyolefinic thermoplastics. For example, an ethylene-propylene copolymer elastomer or terpolymer elastomer can be blended with polypropylene. Depending on the ratio of the elastomer to the polypropylene, the properties of the blend composition can be varied from a high modulus, high hardness grade to a flexible, soft grade. Other modifications to meet specific customer needs may be made by adding other ingredients to produce useful compounds. Examples of such technology are disclosed in U.S. Pat. Nos. 4,013,169 and 4,098,848 both to Morris, U.S. Pat. No. 3,835,201 to Fischer and U.S. Pat. No. 4,220,579 to Rinehart.
Another example of a blend of a thermoplastic and an elastomer to produce a TPO is disclosed in U.S. Pat. No. 3,806,558 to Fischer which discloses the technology of blending an ethylene-propylene-diene terpolymer (EPDM) elastomer with either polyethylene or polypropylene while being dynamically semi-cured by either peroxide or sulfur. Similar blends have also been disclosed in U.S. Pat. No. 4,130,535 to Coran et al.
Other polyolefin materials which can be used in TPO compounds include: linear low density polyethylene, copolymers of ethylene with vinyl acetate (EVA), ethylacrylate (EEA), and methyl acrylate (EMA), the semi-crystalline copolymer of propylene and ethylene and polybutene-1. The rubber also can be ethylene-propylene rubber, however, there will be no unsaturation site for crosslinking.
Another example of blended TPE has been disclosed in U.S. Pat. Nos. 4,130,535 to Coran et al; 4,311,628 to Abdou-Sabet et al and 4,409,365 to Coran et al which discloses a blend of polypropylene and crosslinked ethylene-propylene terpolymer. In spite of the fact that most of the EPDM is completely crosslinked, such blends are manufactured by a unique compounding technique which allows them to be processed as ordinary TPEs. These compounds are manufactured and marketed by Monsanto under the trademark "Santoprene".
Other methods besides blending techniques can also be used to prepare TPEs. One such example is disclosed in U.S. Pat. No. 3,792,127 to Gillies which covers block copolymerization of styrene and butadiene. U.S. Pat. No. 3,265,765 to Holden et al discloses thermoplastic elastomers made by grafting an elastomeric segment, polyisobutylene to polyethylene.
Thermoplastic polyolefin elastomers, similar to unblended polyolefins, often form a crystalline and nonpolar polyolefin-rich surface which also restricts their usefulness in adhesive applications which require good surface wettability to achieve optimal adhesive properties. The inherent characteristic of an olefin based material is its non-porous and non-reactive surface. Articles manufactured from ordinary olefin based compounds do not adhere to metal or other polar substrates without surface modification.
Priming has been the most commonly used method to improve metal adhesion or bonding of olefinic materials. Other pretreatment methods used in industry include solvent etching, sulfuric or chromic acid etching, sodium treatment, ozone treatment, flame treatment, UV irradiation and plasma treatment. However, these pretreatment processes are costly and use hazardous solvents or gases that require extra safety precautions. Furthermore, many of these conventional treatments can cause polymer chain scission, degradation, deformation and oxidation of the finished product. Many of these methods also have become subject to legislative restrictions based on environmental protection.
Recently, there has been an increasing interest by automotive and other industries to explore the possibility of using olefinic TPE compounds which can meet the application performance requirements, and can adhere to metal. It is important that these olefinic compounds can be tailor made to end use specifications and also exhibit exceptional wettability and bondability.